JPH0649033B2 - Nuclear magnetic resonance imaging system - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to nuclear magnetic resonance (hereinafter referred to as N
The present invention relates to an NMR imaging apparatus that visualizes a desired portion of a subject by using (abbreviated as MR).

[0002]

2. Description of the Related Art In an NMR imaging apparatus, an atomic nucleus is irradiated with a high frequency wave to be excited, and a high frequency signal (this is called an NMR signal) emitted from a resonant atomic nucleus is detected. A coil is usually used for irradiation and detection of a high-frequency signal, and a saddle type, a solaid type and various modified coils thereof are considered. Since the irradiation and the detection are performed in different time zones, a method in which both of them are used by one coil is also known. However, in the NMR imaging apparatus for the human body, a relatively large irradiation coil and a relatively small irradiation coil located near the human body are provided in order to realize spatially wide and uniform irradiation and detection with high sensitivity and good SN. A detection coil is often used. When the irradiation coil is arranged in the same direction as the detection coil, both are coupled in high frequency and the detection sensitivity is lowered. Therefore, both are usually arranged on orthogonal axes.

By the way, since the sensitivity of the detection coil directly affects the SN ratio of the reconstructed image, many researches and improvements have been made. The excited spin behaves as if a small magnetic pole piece is rotating on the same plane, so paying attention to this point, CNC (CNC)
HEN) et al. Have proposed a quadrature coil for detection with two orthogonal coil systems, and (CN CHEN) et al.
"J of Magnetic Resonance (JOF
MAGNETIC RESONANCE) "53-32
4-327, 1983) In principle, it is said that the SN ratio is improved by 2 times. FIG. 3 is a diagram showing the principle of this quadrature coil, and a tuning circuit and the like are omitted here for simplification of description. In the figure, the magnetization rotating in one plane induces the same signal in coil 1 and coil 2 with a 90 ° phase difference. Here, since the coil 1 and the coil 2 are orthogonal to each other in the axial direction, signals are detected with random noises independent of each other. Possible sources of noise are the resistance of the coils 1 and 2, the equivalent resistance from the subject due to the magnetic coupling and electrical coupling between the subject and the coils 1 and 2. When the phases of the signals of both coils 1 and 2 are combined by the phase shifter 3 and the like and added by the combiner 4, the signal is doubled and the noise is multiplied by √2, and as a result, the SN ratio is improved by √2.

However, this result is established when the sensitivities of the coil 1 and the coil 2 are equal. For this purpose, the coils 1 and 2 have the same size and shape, and the equivalent resistance from the subject is also equal. There is a need. The equivalent resistance from the subject does not actually become equivalent due to the difference in the shape of each subject, and in reality an imbalance occurs between the orthogonal coils 1 and 2. Further, as shown in FIG. 4, when the solenoid type coil 5 which is said to have the highest sensitivity is adopted on the one axis side in a shape close to the subject 6, the same sensitivity is practically obtained on the axis side orthogonal to it. It is not possible to set a coil with a coil, and it becomes a saddle type coil 7, for example, and the unbalance between both coils 5 and 7 becomes larger and larger. As a result, the SN ratio of both coils 5 and 7 actually reaches 1.4 to 1.8 times, for example. Further, since the coils 1 and 2 or the coils 5 and 7 have spatially different sensitivity characteristics, it is not possible to satisfy the above optimum condition (the coils 1, 2, 5 and 7 have the same sensitivity) in a wide area. .

[0005]

In the prior art, no consideration was given to the imbalance and the SN ratio finally obtained was low. For example, in FIG. 4, when the SN ratio of the coil 5 is 80 and the SN ratio of the coil 7 is 50,
When the signal levels of the both were aligned and added, the final SN ratio was 84.8, which was only an improvement of 6%. Therefore, there is a problem that a high quality image cannot be obtained.

The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a nuclear magnetic resonance imaging apparatus having a high SN ratio and capable of obtaining a high quality image.

[0007]

The above-mentioned problem is that information having different SN ratios are added with the same weighting.
Therefore, in the present invention, the signals are weighted according to the difference in the amount of information between the two, so that a higher SN ratio can be achieved.

In FIG. 4, the signal component obtained by the coil 5 is S ₁ , the SN ratio is SN ₁ , and the coil 7 is also S ₂ , SN ₂ . If the absolute sensitivity ratio S ₂ / S ₁ is G and the weighted addition ratio is 1 on the coil 5 side and K on the coil 7 side, the SN ratio after addition is

[0009]

[Equation 1]

[0010] Here, the condition for maximizing the SN ratio is

[0011]

[Equation 2]

The SN ratio SNmax at that time is

[0013]

[Equation 3]

[0014]

When the above equation (2) is applied while paying attention to the difference in the amount of information in consideration of the imbalance and the addition is performed by the equation (3), for example, the SN ratio can be increased to 94.3,
It will be improved by about 18%.

The optimization of the SN ratio by the equation (2) can be applied by the following means. That is, it is a means for measuring the SN ratio near the center of each quadrature coil, using the value in equation (2) to obtain K, and adjusting the gain of the output of at least one coil using a semi-fixed attenuator. .

[0017]

According to the SN ratio of the output of each coil, at least one of the outputs is weighted by the attenuator, the phase difference is corrected by the phase shifter, and the outputs are added to increase the final SN ratio. Image is obtained.

[0018]

Embodiments of the present invention will now be described in detail with reference to the accompanying drawings.

FIG. 1 is a block diagram showing an example of the overall configuration of a nuclear magnetic resonance imaging apparatus according to the present invention. This nuclear magnetic resonance imaging apparatus obtains a tomographic image of a subject 6 using a nuclear magnetic resonance (NMR) phenomenon, and includes a static magnetic field generating magnet 10 and a central processing unit (hereinafter referred to as CPU) 1
1, a sequencer 12, a transmission system 13, a magnetic field gradient generation system 14, a reception system 15 and a signal processing system 16.

The static magnetic field generating magnet 10 is for generating a strong and uniform static magnetic field around the subject 6 in the body axis direction or in the direction orthogonal to the body axis. Permanent magnet type, normal conducting type or superconducting type magnetic field generating means is arranged in the space provided.

The sequencer 12 operates under the control of the CPU 11 and sends various commands necessary for collecting tomographic image data of the subject 6 to the transmission system 13, the magnetic field gradient generation system 14 and the reception system 15.

The transmission system 13 includes a high frequency oscillator 17, a modulator 18, a high frequency amplifier 19, and a high frequency coil 2 on the transmission side.
0a, and the high frequency pulse output from the high frequency oscillator 17 is modulated by the modulator 1 according to the instruction of the sequencer 12.
8, the amplitude-modulated high-frequency pulse is amplified by the high-frequency amplifier 19 and then supplied to the high-frequency coil 20a arranged in proximity to the subject 6, whereby the subject 6 is irradiated with electromagnetic waves. It is like this.

The magnetic field gradient generation system 14 is composed of a gradient magnetic field coil 21 wound in three axial directions of X, Y and Z and a gradient magnetic field power source 22 for driving each coil, and according to an instruction from the sequencer 12. By driving the gradient magnetic field power source 22 of each coil, the gradient magnetic fields Gx, Gy, and Gz are applied to the subject 6 in the triaxial directions of X, Y, and Z. The slice plane for the subject 6 can be set by the method of applying the gradient magnetic field.

The receiving system 15 is composed of a high frequency coil 20b on the receiving side, an amplifier 23, a quadrature phase detector 24 and an A / D converter 25, and an object to be examined by electromagnetic waves emitted from the high frequency coil 20a on the transmitting side. 6 response electromagnetic wave (N
The MR signal) is detected by the high-frequency coil 20b arranged close to the subject 6, and the amplifier 23 and the quadrature detector 2 are detected.
4 is input to the A / D converter 25 to be converted into a digital amount, and is further converted into two series of collected data sampled by the quadrature phase detector 24 at the timing according to the instruction from the sequencer 12, and the signal is a signal. The data is sent to the processing system 16.

The signal processing system 16 includes a CPU 11, a recording device such as a magnetic disk 26 and a magnetic tape 27, and a C
It is composed of a display 28 such as an RT and the CPU 11
Fourier transform, correction coefficient calculation image reconstruction, and the like are performed, and the signal intensity distribution of an arbitrary cross section or the distribution obtained by performing an appropriate calculation on a plurality of signals is imaged and displayed on the display 28. ing.

In FIG. 1, the high frequency coils 20a and 20b and the gradient magnetic field coil 21 on the transmitting and receiving sides are
Static magnetic field generating magnet 10 arranged in the space around the subject 1.
It is located in the magnetic field space.

Here, the high frequency coil 20b according to the present invention.
Are orthogonal coils 5 and 7 as shown in FIG. Further, as described above, the high-frequency coil 20a for irradiation may also be used as the receiving coil on the uniaxial side, and the receiving-only coil orthogonal thereto may be provided to configure the orthogonal coils 5, 7. Further, both the coil 5 and the coil 7 in FIG. 4 may be used for both irradiation and reception. 2 orthogonal in each case
It is important to extract the signal from the two coils 5 and 7.

FIG. 2 shows an embodiment of the main part of the present invention described above. The inputs 30 and 31 are connected to the outputs of the quadrature coils 5 and 7 tuned to the resonance frequency. The respective signals are amplified by the preamplifiers 32 and 33, and one of the preamplifiers 3
The output of 3 is passed through a semi-fixed attenuator 34 and a phase shifter 35 for making the phases of both outputs the same, and then added by the output of the other preamplifier 32 by an adder 36 and output to an output 37.

The semi-fixed attenuator 34 is an SN of each output when a reference object (not shown) is measured in advance.
The ratio and the absolute sensitivity ratio G are set to the values obtained by using the equation (2).

Since the intent of the present invention is to adjust the relative sensitivity balance obtained by the equation (2), the same thing can be implemented by modifying the configuration illustrated in FIG.
For example, the arrangement order of the circuit units 33 to 35 is changed, a variable gain amplifier in which the circuit units 33 and 34 are integrated is used, and the impedances of the coils 5 and 7 after being tuned are changed and combined. .

In any of the systems, according to this embodiment, it is possible to improve the SN ratio in the vicinity of the center, which is most important in image quality.

[0032]

As described above, the present invention considers the imbalance in characteristics that occurs between the detecting means (quadrature coils) for detecting the NMR signal on each of two orthogonal axes.
Since the MR signals are weighted, the SN ratio can be increased, and a good quality image can be obtained.

[Brief description of drawings]

FIG. 1 is a block diagram showing an example of the overall configuration of a device of the present invention.

FIG. 2 is a block diagram showing a configuration example of a main part of the same apparatus.

FIG. 3 is a principle diagram of a quadrature coil used for NMR signal detection of an NMR imaging apparatus.

FIG. 4 is a perspective view showing a specific example of the same coil.

[Explanation of symbols]

5, 7 ... Quadrature coil, 6 ... Subject, 10 ... Static magnetic field generating magnet, 11 ... Central processing unit (CPU), 13 ... Transmission system, 1
4 ... Magnetic field gradient generating system, 15 ... Receiving system, 16 ... Signal processing system, 20b ... High frequency coil (NMR signal detecting means), 3
4 ... Attenuator, 35 ... Phase shifter, 36 ... Adder.

─────────────────────────────────────────────────── ─── Continuation of the front page (51) Int.Cl. ⁵ Identification code Internal reference number FI Technical display location 8932-4C A61B 5/05 374 9219-2J G01N 24/02 N 9219-2J 24/08 D

Claims (1)

[Claims] 1. A means for applying a static magnetic field and a gradient magnetic field to a subject, a means for applying a high frequency to cause nuclear magnetic resonance in atomic nuclei of atoms constituting the tissue of the subject, and a signal by the nuclear magnetic resonance. In a nuclear magnetic resonance imaging apparatus comprising nuclear magnetic resonance signal detection means for respectively detecting X and Y, and arithmetic means for performing image reconstruction calculation using the nuclear magnetic resonance signals. An attenuator capable of weighting at least one of the two nuclear magnetic resonance signals according to the SN ratio of the two nuclear magnetic resonance signals detected by the means, and a phase shifter for correcting the phase difference between the two nuclear magnetic resonance signals And an adding means for adding the two weighted and phase difference-corrected nuclear magnetic resonance signals.